As is well known, various processes and catalysts exist for the homopolymerization or copolymerization of olefins. For many applications it is of primary importance for a polyolefin to have a high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, when accompanied by a narrow molecular weight distribution, provides a polyolefin or an ethylene-α-olefin copolymer with high strength properties.
Traditional Ziegler-Natta catalyst systems—a transition metal compound cocatalyzed by an aluminum alkyl—are capable of producing polyolefins having a high molecular weight but a broad molecular weight distribution.
More recently a catalyst system has been developed wherein the transition metal compound has two or more cyclopentadienyl ring ligands, such transition metal compound being referred to as a metallocene—which catalyzes the production of olefin monomers to polyolefins. Accordingly, metallocene compounds of the Group IV B metals, particularly, titanocene and zirconocene, have been utilized as the transition metal component in such “metallocene” containing catalyst system for the production of polyolefins and ethylene-α-olefin copolymers. When such metallocenes are cocatalyzed with an aluminum alkyl—as is the case with a traditional type Ziegler-Natta catalyst system—the catalytic activity of such metallocene catalyst system is generally too low to be of any commercial interest.
It has since become known that such metallocenes may be cocatalyzed with an alumoxane—rather than an aluminum alkyl—to provide a metallocene catalyst system of high activity which catalyzes the production of polyolefins.
A wide variety of Group IV B transition metal compounds of the metallocene type have been named as possible candidates for an alumoxane cocatalyzed catalyst system. Hence, although bis(cyclopentadienyl) Group IV B transition metal compounds have been the most preferred and heavily investigated type metallocenes for use in metallocene/alumoxane catalyst for polyolefin production, suggestions have appeared that mono and tris(cyclopentadienyl) transition metal compounds may also be useful. See, for example, U.S. Pat. Nos. 4,522,982; 4,530,914 and 4,701,431. Such mono(cyclopentadienyl) transition metal compounds as have heretofore been suggested as candidates for a metallocene/alumoxane catalyst are mono(cyclopentadienyl) transition metal trihalides and trialkyls.
More recently International Publication No. WO 87/03887 has appeared which describes the use of a composition comprising a transition metal coordinated to at least one cyclopentadienyl and at least one heteroatom ligand as a metallocene type component for use in a metallocene/alumoxane catalyst system for α-olefin polymerization. The composition is broadly defined as a transition metal, preferably of Group IV B of the Periodic Table which is coordinated with at least one cyclopentadienyl ligand and one to three heteroatom ligands, the balance of the coordination requirement being satisfied with cyclopentadienyl or hydrocarbyl ligands. The metallocene/alumoxane catalyst system described is illustrated solely with reference to transition metal compounds which are bis(cyclopentadienyl) Group IV B transition metal compounds.
Even more recently, at the Third Chemical Congress of North America held in Toronto, Canada in June 1988, John Bercaw reported upon efforts to use a compound of a Group III B transition metal coordinated to a single cyclopentadienyl heteroatom bridged ligand as a catalyst system for the polymerization of olefins. Although some catalytic activity was observed under the conditions employed, the degree of activity and the properties observed in the resulting polymer product were discouraging of a belief that such transition metal compound could be usefully employed for commercial polymerization processes.
A need still exists for discovering catalyst systems that permit the production of higher molecular weight polyolefins and desirably with a narrow molecular weight distribution.